In the exploration and production of hydrocarbons, it is sometimes necessary to enlarge a borehole. The type of tool utilized to ream the borehole to a larger diameter is commonly called an "underreamer".
Underreaming a borehole is necessary, for example, when drilling through fast moving formations such as salt or sloughing shale. Fast moving formations can creep into the borehole, thereby decreasing the diameter of the borehole. The reduction of the size of the borehole can cause a hang-up of the drillstring. If a hang-up occurred, the drillstring could twist off and separate. This would delay drilling and require the use of complex tools to recover the separated section of drillstring. Also, the reduction of the diameter of the borehole can prevent the removal of a bottom-hole assembly from the borehole. The underreaming of the borehole will allow wells penetrating fast moving formations to be drilled more easily and effectively.
Underreaming is also desirable when drilling through formations which require multiple casing to be set within the borehole (such as very deep wells). Each time a casing is set, the diameter of borehole is reduced. To minimize this reduction in the borehole diameter, it is desirable to use casings whose outside diameter is very close to the inside diameter of the previous casing set. This, however, reduces the space available for cement. The subsequent reduction in the amount of cement between the casing and the borehole wall is undesirable.
In order to provide adequate space between the new casing and the borehole for adequate cement, the borehole below the level of the old casing should be enlarged.
Underreaming can also be used advantageously to enlarge the diameter of the borehole in the region from which hydrocarbons are recovered. The enlarged borehole will provide enhanced recovery of hydrocarbons and will also allow for the installation of a gravel pack or other apparatus which can enhance the recovery of hydrocarbons from the producing reservoir.
To underream a section of a formation, an underreaming tool must be run into the previously drilled section of the borehole to the designated depth. The underreaming tool must then be operated to underream the selected portion of the underground formation. Once the desired portion of the underground formation is underreamed, the underreaming tool must be removed from the borehole. This requires the tool be capable of being passed through a section of borehole which is smaller in diameter than the enlarged borehole which the tool must create.
In an attempt to provide a tool which satisfies the above requirements, several designs have been developed. In one type of design, the region carrying the cutting elements has a geometric center which is offset from the center of rotation of the tool. Once this type of tool is lowered to the area of the formation to be underreamed, it is rotated in the hole. Due to the offset design of the tool, a borehole is produced which has a larger diameter than the borehole through which the tool was lowered. This type of design is exemplified by the tool disclosed in U.S. Pat. No. 3,851,719, to Thompson et al. One limitation of tools of this design is that they are unstable and inefficient. Also, tools of this design are subject to a phenomenon known as "whirl".
Whirl occurs on an underreamer when it does not rotate smoothly about its central axis of rotation. This is a result of the radial imbalance forces created during the cutting process. These imbalance forces cause the instantaneous center of rotation of the underreamer to become some point other than the centerline of the borehole. As the underreamer rotates, the instantaneous center of rotation changes relative to the centerline of the borehole. This causes the underreamer to move laterally or whirl around the borehole. When the tool whirls, the center of rotation can change randomly relative to the centerline of the borehole. Alternatively, the movement of the center of rotation of the tool relative to the centerline of the borehole may follow a regular pattern, such as the pattern Created by a tool exhibiting "backward whirl". Backward whirl occurs when the frictional contact between the cutting elements and the wellbore cause the underreamer to roll counter-clockwise around the surface of the wellbore as the underreamer rotates in a clockwise direction. The whirling process is regenerated because of the friction which is always generated between the cutting elements of the underreamer and the borehole wall and because of centrifugal forces which continually act on the underreamer.
A cutting element on an underreamer exhibiting whirl is subject to increased impact loads which at times are directed in a reverse or sideways direction from that which would be expected for the designed direction of travel. These increased impact loads cause increased wear and breakage of the cutting elements.
Other tools, such as the one disclosed in U.S. Pat. No. 5,060,738 to Pitlard et al., utilize Cutting elements which are carried on extendable arms. With the arms in a retracted position, the underreamer may be run into the existing borehole and then extended to a projected position. As the drillstring to which the tool is attached is rotated, the cutting elements cut into the sidewall of the formation to expand the radius of the borehole. With the arms in a raised position, the desired section of the borehole is underreamed to a larger diameter. The arms are then retracted and the underreamer is removed from the borehole.
Studies have shown that underreamers, such as disclosed in the Pittard et al. patent, are also unstable and subject to whirl, due to circumferential drilling imbalance forces which act on this type of underreamer. Various methods have been utilized in an attempt to improve the performance of underreamers which have expandable arms. The methods include reducing the reaming speed and dynamically balancing the underreamer and lower drillstring. Developers have typically focused on building more robust underreaming tools. It was hoped that by building more robust tools, cutter and arm breakage could be prevented or at least minimized. This philosophy has resulted in designs which utilized one-piece arms versus two-piece arms, designs with increased cutter density to limit individual cutter loading, designs in which the cutter profile has been modified in an attempt to limit cutter loading, and designs in which the cutters have been placed in a position that will limit individual cutter loading.